Optical disks have excellent removability/portability and random access performance. Therefore, it has become more and more prevalent to employ optical disks as memories in various information equipment fields, e.g., personal computers. As a result, there has been an increasing demand for increasing the recording capacitance of optical disks.
In general, guide grooves for tracking control purposes are formed on rewritable optical disks, so that data is recorded and reproduced by utilizing the guide grooves as tracks. In addition, a track is divided into a plurality of sectors for sector-by-sector management of data. Therefore, in the production of such disks, address information for each sector is often formed in the form of pits while forming the guide grooves.
In currently prevalent rewritable optical disks, tracks for recording data are either the grooves formed during the disk formation (grooves) or the interspaces between grooves (lands). On the other hand, optical disks of a land-groove recording type for recording data on both the grooves and the lands have also been proposed.
FIG. 22 illustrates an exemplary optical disk of the land-groove recording type. As used herein, the portions which are located closer to the optical disk surface are referred to as "grooves"; whereas the portions which are located further away from the optical disk surface are referred to as "lands", as shown in FIG. 22. It should be noted that "lands" and "grooves" are mere names; therefore, the portions which are located closer to the optical disk surface may be referred to as "lands", while the portions which are located further away from the optical disk surface may be referred to as "grooves".
An optical disk of the land-groove recording type requires sector addresses for both the lands and the grooves. In order to facilitate the process of forming address pits on an optical disk, an intermediate address method has been studied in which address pits are formed between a land and a groove adjoining each other so that the same address is shared by the adjoining tracks (Japanese Laid-Open Publication No. 6-176404).
Hereinafter, the intermediate address, a tracking control method for reading information from an optical disk, and a method for reading signals from an intermediate address will be described with reference to the figures.
FIG. 23 is a schematic diagram showing an optical disk having a sector structure. In FIG. 23, reference numeral 200 denotes a disk; reference numeral 201 denotes a track; reference numeral 202 denotes a sector; reference numeral 203 denotes a sector address region; and reference numeral 204 denotes a data region. FIG. 24 is a magnified view of a sector address region schematically showing a conventional intermediate address. In FIG. 24, reference numeral 206 denotes address pits; reference numeral 207 denotes recording marks; 208 denotes a groove track; reference numeral 209 denotes a land track; and reference numeral 210 denotes a light spot.
In the optical disk shown in FIG. 24, the groove 208 and the land 209 are employed as tracks. Data signals can be recorded by forming the recording marks 207 on the groove 208 and the land 209. The groove track 208 and the land track 209 have the same track pitch Tp. The center of each address pit 206 is shifted by Tp/2 from the center of the groove track 208 along the radius direction. In other words, each address pit 206 is centered around the boundary between the groove 208 and the land 209. Although the lengths or intervals of the address pits 206 are modulated by an address signal, FIG. 24 only schematically illustrates the shapes of the address pits 206.
FIG. 25 is a block diagram showing the conventional tracking control and the signal processing for reading signals on an optical disk.
The structure shown in FIG. 25 will be described below, In FIG. 25, reference numeral 200 denotes a disk; reference numeral 201 denotes a track; reference numeral 210 denotes a light spot; and reference numeral 211 denotes a disk motor for rotating the disk 200. An optical head 212 optically reproduces a signal on the disk 200. The optical head 212 includes a semiconductor laser 213, a collimation lens 214, an object lens 215, a half mirror 216, photosensitive sections 217a and 217b, and an actuator 218. A tracking error signal detection section 220 detects a tracking error signal indicating the amount of dislocation between the light spot 210 and the track 201 along the radius direction. The tracking error signal detection section 220 includes a differential circuit 221 and a LPF (low pass filter) 222. A phase compensation section 223 generates a drive signal from a tracking error signal for driving the optical head. A head driving section 224 drives the actuator 218 in the optical head 212 in accordance with the drive signal.
An address reproduction section 234 includes an addition circuit 225, a waveform equalization section 226, a data slice section 227, a PLL (phase locked loop) 228, an AM detection section 229, a demodulator 230, a switcher 231, and an error detection section 232. The addition circuit 225 adds signals from the photosensitive sections 217a and 217b. The waveform equalization section 226 prevents the inter-sign interference of a reproduced signal. The data slice section 227 digitizes the reproduced signal at a predetermined slice level. The PLL (Phase Locked Loop) 228 generates a clock which is in synchronization with the digitized signal. The AM detection section 229 detects AMs (address marks). The demodulator 230 demodulates the reproduced signal. The switcher 231 separates the demodulated signal into data and an address. The error detection section 232 performs an error determination in the address signal. An error correction section 233 corrects errors in the data signal.
Hereinafter, an operation for tracking control will be described. Laser light radiated from the semiconductor laser 213 is collimated by the collimate lens 214 and converged on the disk 200 via the object lens 215. The laser light reflected from the disk 200 returns to the photosensitive sections 217a and 217b via the half mirror 216, whereby the distribution of light amount is detected as an electric signal, which is determined by the relative positions of the light spot 210 and the track 201 on the disk. In the case of using the two-divided photosensitive sections 217a and 217b, a tracking error signal is detected by detecting a difference between the photosensitive sections 217a and 217b by means of the differential circuit 221 and extracting a low frequency component of the differential signal by means of the LPF 222. In order to ensure that the light spot 210 follows the track 201, a drive signal is generated in the phase compensation section 223 such that the tracking error signal becomes 0 (i.e., the photosensitive sections 217a and 217b have the same distribution of light amount), and the actuator 218 is moved by the head driving section 224 in accordance with the drive signal, thereby controlling the position of the object lens 215.
On the other hand, when the light spot 210 follows the track 201, the amount of reflected light is reduced at the recording marks 207 and at the address pits 206 on the track owing to interference of light, thereby lowering the outputs of the photosensitive sections 217a and 217b, whereas the amount of reflected light increases where pits do not exist, thereby increasing the outputs of the photosensitive sections 217a and 217b. The total light amount of the output from the photosensitive sections which corresponds to the recording marks 207 and address pits 206 is derived by the addition circuit 225, led through the waveform equalization section 226 so as to remove the inter-sign interference of the reproduced signal, and digitized at a predetermined slice level at the data slice section 227 so as to be converted into a signal sequence of "O" and "1". Data and a read clock are extracted from this digitized signal by the PLL 228. The demodulator 230 demodulates the recorded data which has been modulated, and converts it into a data format which allows for external processing. If the demodulated data is a signal in the data region, the errors in the data are corrected in the error correction section 233, whereby a data signal is obtained. On the other hand, if the AM detection section 229 detects an AM signal for identifying the address portions in a signal sequence that is constantly output from the PLL 228, the switcher 231 is switched so that the demodulated data is processed as an address signal. The error detection section 232 determines whether or not the address signal which has been read includes any errors; if no error is included, the address signal is output as address data.
FIG. 26 shows the states of a reproduced signal (RF signal) and a tracking error signal (TE signal) when the light spot 210 passes the sector address region 203 in the above-described configuration. Although the light spot 210 is on the center of the track in the data region 204, a drastic dislocation occurs between the light spot 210 and the address pits 206 immediately after the light spot 210 enters the sector address region 203, thereby greatly fluctuating the level of the TE signal. The light spot 210 cannot rapidly follow the address pits but gradually comes closer to the address pits, as indicated by the broken line. However, since the sector address region 203 is short and the data region 205 (which is a grooved region) is reached before the light spot 210 manages to completely follow the address pits, a tracking control is performed so that the off-tracking becomes zero in the grooved region. The amount of off-tracking in the last portion of the sector address region is defined as Xadr. Moreover, since a portion of the light spot 210 is on the address pits 207, an RF signal as shown in FIG. 26 is obtained. The RF signal amplitude Aadr varies in accordance with the distance between the light spot 210 and the address pits 206. Specifically, Aadr decreases as the distance becomes larger, and increases as the distance becomes smaller.